What adjective would describe a task that is demanding great effort or labor?

The adjective that best describes a task that requires great effort or labor is "arduous." This term is often used to convey the difficulty and strenuous nature of a task, indicating that it demands significant physical or mental effort to accomplish. For example, climbing a mountain or completing an extensive research project can be described as arduous, as they both involve considerable challenges and persistence.

The other terms provided do not convey the same meaning. "Arid" refers to a dry or barren condition, typically related to climate, and does not have implications of effort. "Apathetic" describes a lack of interest or enthusiasm, which is unrelated to the concept of effort or labor. "Apt" relates to being appropriate or suitable, but again, it does not convey the sense of demanding effort. Hence, in the context of describing a laborious task, "arduous" is the most appropriate choice.